In any conventional ABS system in use on vehicles today, there is generally provided an electric motor-driven opposed piston-type pumping unit for supplying pressurized brake fluid to any one of the brakes on instant, automatically controlled demand. The electrically-driven motor is a part of the ABS unit and includes a driveshaft which will include one or more cam lobes. Transversely disposed of the driveshaft, generally at 180.degree. from one another, are a pair of pumping pistons within a respective pump body. The output of each pumping piston provides high pressure brake fluid, generally in the order of 2500 psi, to a respective segment of the braking system. In other words, in a conventional braking system for a four-wheel vehicle, the braking system is divided into two separate braking segments beginning at a dual-chambered master cylinder. Each segment provides the braking force to a predetermined pair of wheels. One segment may be to the left front-right rear wheels, for example, while the other segment provides the braking power to the right front-left rear wheels. In this way, the vehicular braking system is generally considered fail-safe in that if braking pressure were for any reason to be lost to one segment or pair of wheels, the remaining half of the brake system would be functional and sufficient to bring the vehicle under control. The ABS system is typically integrated into the vehicular braking system in the same manner, thereby providing anti-lock braking characteristics from each of the pair of pumping units to a respective one of the braking system segments. Likewise, traction assist capabilities are commonly added to the ABS system in such a manner as to use the pump output for applying brake pressure to a wheel or wheels to preclude wheel slip. This means increased use of the common pump, and added concern for the cost, performance attributes of the pump.
Due to the performance demanded of the ABS unit, it is important that the pumping unit provide high pressure brake fluid to the system as efficiently and quickly as possible.
Further, it is important that manufacturing costs be minimized and that performance reliability be maximized.
It is also important that the overall noise or decibel levels of the pump when it's running be kept to a minimum so as to be imperceptible to the operator. That is, while braking situations requiring employment of the ABS/TA system are quite rare, it is common practice to provide a diagnostic check of the system including operating momentarily the hydraulic pumps, each time the vehicle is started. Obviously, it is important that the operator of the vehicle, when first starting the vehicle, not be concerned in any way with noise generated from the brake diagnostic checking system.